Greenhouse Temperature Calculator

Greenhouse temperature management involves balancing solar heat gain through glazing, heat loss through the structure, ventilation for cooling, and supplemental heating for cold nights. Getting this right prevents crop damage from both overheating and frost.

Heat Loss formula (for minimum heating needs): Heat Loss (BTU/hr) = U-value × Surface Area × (Indoor Temp − Outdoor Temp)

Ventilation for cooling (air changes method): CFM Required = (Greenhouse Volume × Air Changes per Hour) / 60

What each variable means:

• U-value — the overall heat transfer coefficient of the glazing; U = 1/R-value
  • Single glass: U ≈ 1.13 BTU/hr·ft²·°F | Double glass: U ≈ 0.55 | Twin-wall polycarbonate: U ≈ 0.62
• Surface Area — total glazed area (walls + roof), NOT floor area; includes any thermal mass walls
• ΔT — temperature difference between inside target (e.g., 50°F night minimum) and design outdoor low temperature
• CFM — cubic feet per minute of air exchange required to prevent overheating
• Air Changes per Hour — typically 40–60× for greenhouses in summer (much higher than buildings, because solar gain is intense)

Worked example — Heating: Greenhouse: 12’ × 20’ footprint, 10’ ridge height. Twin-wall polycarbonate glazing, U = 0.62. Surface area (approximate): roof = 2 × (10’ × 20’) = 400 ft², two long walls: 2 × (12’ × 8’) = 192 ft², two gable ends: 2 × (½ × 12 × 5) = 60 ft² Total ≈ 652 ft² Design outdoor low: 20°F. Target indoor: 50°F. ΔT = 30°F. Heat Loss = 0.62 × 652 × 30 = 12,127 BTU/hr Heater needed: 12,500–15,000 BTU/hr (round up 20% for infiltration)

Worked example — Ventilation: Same greenhouse volume: 12 × 20 × 8 (avg height) = 1,920 ft³ At 45 air changes/hour: CFM = (1,920 × 45) / 60 = 1,440 CFM Use two 750 CFM exhaust fans, controlled by a thermostat set to open at 85°F (29°C).

Temperature thresholds by crop:

• Tropical plants: 60°F (15°C) minimum | Cool-season vegetables: 35–40°F | Orchids: 55–65°F